The objective of the proposed program is to utilize the recently developed method of stable isotope enrichment and the concept of simultaneous double labeling to study some fundamental features of selenium nutrition and metabolism directly in healthy human adults in order to acquire the necessary, and at present missing, data base for later application to human nutrition and metabolism of selenium under unhealthful physiologic and dietary conditions. The parameters of nutrition and metabolism to be measured directly with this novel tracer methodology are gastrointestina absorption, urinary excretion (quantitative and dynamics), longterm body retention, nature and quantitative significance of urinary metabolites, and the first pass effect of the liver. The dietary and host factors to be investigated are: dietary forms of selenium and body selenium status.